In total knee arthroplasty, the convention is to resect the entire proximal tibia to create a plateau surface on which a tibial base prosthesis can be implanted. Such conventional resection techniques typically sacrifice one or both of the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL) since the resections removed the bony attachment site for those ligaments (the “tibial eminence”). Often, PCL and ACL functions are replaced by the prosthesis, which may utilize a stabilizing post on the tibial insert and a corresponding receptacle on the femoral component or increased sagittal conformity. While these prostheses generally restore anterior-posterior stability, they may not feel as “natural” as a normal knee and are less tissue-conserving.
If any one or both of the cruciate ligaments are salvageable, it is sometimes desirable (especially for young and active patients) to conserve either or both the ACL and PCL, in order to preserve natural biomechanics, range of motion, and feeling.
In current PCL-sparing knee implants, a posterior portion of the tibial insert and/or tibial base member may have a slight cut-out to provide space for the PCL and its attachment site on a remaining portion of the tibial eminence. A surgeon must remain careful not to resect portions of bone adjacent the PCL attachment areas. The ACL is generally sacrificed when using these so-called posterior cruciate-retaining prostheses.
Alternatively, a surgeon may attempt to preserve both the ACL and PCL, which is sometimes accomplished by installing two unicondylar implants. The tibial eminence and cruciate ligaments attached thereto are left intact. The medial and lateral tibial plateau areas are resected and replaced with separate unicondylar tibial trays and corresponding inserts. One disadvantage of implanting two separate unicondylar implants includes the difficulty in properly aligning the two implants in relation to each other. If the two implants are not aligned properly, wear may be accelerated, mechanical axis alignment may be compromised, and femoral motion may feel unnatural to the patient. Surgical implantation time may also be increased due to the added complexity of installing two implants instead of one.
In lieu of two separate unicondylar implants, surgeons have the alternative option of preserving both the ACL and PCL by implanting a single bi-cruciate retaining implant, which comprises a single tibial bearing member (which may be an insert) and/or tibial base member. Prior art bi-cruciate retaining implants are essentially formed of an insert and a base member, each having two unicondylar portions joined by a thin anterior bridge which connects the two. The thin anterior bridges may fail to support the high torsional loading experienced by active patients, and past implants have been known to eventually bend or shear in half over time, requiring premature revision surgery. Even minor bending and shearing experienced by such prior art devices may reduce performance and eventually cause loosening or de-laminating of the implant from the bone on either or both of the medial and lateral sides.
Additional problems with prior bi-cruciate retaining designs include fracture of the bone adjacent to the area connecting the ACL to the tibia (i.e., the anterior tibial eminence). Such fractures are especially common when bone portions anterior to the ACL attachment point are removed in order to provide enough space for the medial and lateral side portions to be connected by said thin anterior bridge.